Beyond Bait: Part 1 of Earthworms in agriculture

For some it might seem insignificant, but many may agree that there are few things as exciting as digging up a lump of soil and find it wriggling with earthworms! Whether serving as bait when fishing in the farm dam or recycling nutrients in your soil, the little wrigglers play an important part in our ecosystem. In this blog, we will find out more about these small invertebrates, their function in our soil and in the following blog, how farming practices influence their presence.

Earthworms belong to the phylum Annelida and are cylindrical segmented worms. They are classified in three different groups according to their behaviour:

1. Epigeic earthworms: Live on or very close to the soil surface, where they feed on rotting plant and animal material. They do not dig tunnels. The commonly known red wigglers fall into this category. One will often find these in compost piles and worm farms, but seldom in the field.
2. Endogeic earthworms: Ingest soil particles to absorb nutrients from decomposed and degraded organic material. They live in the top 20-30 cm of the soil body, where they form horizontal tunnels.
3. Anecic earthworms: Come to the surface during the night to feed on decaying plant material and deposit their casts. During the day, these earthworms dig deep within the soil to form permanent vertical tunnels.

Earthworms in agriculture

Although farming and management approaches differ from farm to farm, we can agree that earthworms should form an integral part thereof. While the three groups of earthworms are of equal importance, at farm level their importance may differ depending on farming practices. Where farmers make use of Johnson-Su bioreactors, the use epigeic earthworms is of utmost importance. On the other hand, the other two groups are very important in the field, where these little creatures truly start adding value to your farming operation with no additional cost to the farmer.

Earthworms can be seen as soil conditioners as their presence influences several soil properties that determine the overall soil health. Their burrowing activity improves soil structure by mixing soil layers and binding the soil with organic matter. Simultaneously, earthworms recycle dead organic material by ingesting and digesting it together with soil particles and forming humus. This process of chewing through the soil makes nutrients water soluble and more readily available for plant uptake.

Importance of these little guys in sustainable agriculture:

• Improved infiltration = reduced runoff and erosion. Water fills up the soil profile - the most efficient water storage system on any farm.
• Increased aeration = stimulates root growth and soil microbial activity. Reduces anaerobic conditions resulting in reduced stress.
• Breakdown of organic matter = carbon sequestration and many other advantages of increased soil organic carbon.
• Aggregate stability = improved soil structure, reduced compaction and surface crusting due to slaking.

Let's eat!

Some earthworm species (epigeic) such as red wigglers mainly depend upon intact organic waste for nutrition, whereas other species (endogeic and anecic) appear to prefer organic matter in an advanced stage of decomposition. This is where the symbiotic relationship between soil microbial communities of healthy soils and earthworms comes into play. Before the earthworm species can start feeding, plant residue and organic matter in the soil needs to be broken down to some degree by soil microbes. As soon as this has been accomplished, mineralisation of nutrients commences during digestion in the earthworm’s gut. The digestive system of earthworms consists of a pharynx, oesophagus and gizzard, followed by an anterior intestine that secretes enzymes. The secretion of enzymes through the digestive system creates a dramatic increase in microorganisms. These microorganisms are then spread throughout the soil profile during tunnelling and expulsion. They inoculate the soil and in turn help to provide food for the earthworms. This symbiotic relationship not only helps to provide plant-available nutrients, but also stimulates plant growth.

The symbiotic relationship continues to include the farmer. Just as a farmer’s son depends on him to provide juicy earthworms through sustainable farming as bait for his fishing, so the farmer depends on the earthworms to improve his soil, stimulate plant growth and increase yields as a result.

Stay tuned for part 2 of earthworms in agriculture, where we will discuss the silent threats to earthworm populations in our soils.